Explanation
The option within the ISC DHCPD configuration file that defines the IPv4 DNS server address(es) to be sent to the DHCP clients is domain-name-servers. This option takes one or more IPv4 addresses as arguments and specifies the DNS servers that the client should use for name resolution. The domain-name-servers option is part of the standard DHCP options defined in RFC 2132 and is supported by the ISC DHCP server. The domain-name-servers option can be specified globally, per subnet, per shared network, per group, or per host in the dhcpd.conf file. For example, the following line in the dhcpd.conf file will assign the DNS servers
192.168.1.1 and 192.168.1.2 to all DHCP clients:
option domain-name-servers 192.168.1.1, 192.168.1.2;
References:
ISC DHCP 4.4 Manual Pages - dhcp-options
[RFC 2132 - DHCP Options and BOOTP Vendor Extensions]
isc-dhcp-server - Community Help Wiki - Official Ubuntu Documentation